To prevent unauthorized copying of a document composed of image data, an image of a specific pattern (hereinafter, specific image information) such as a background pattern is added to or combined with the document. This specific image information is also referred to as copy prohibiting information or copy preventing information. In addition to this, by implanting various types of management information such as date of a serial number of a copying machine, a document manager, and a copy available limit expiry date into the image data of the document as hidden information, management is made possible.
The specific image information described above is, in some case, attached to the document in such a state as to be indiscernible to a user and in other case, attached to the document in such a state as to be sufficiently discernible to the user as the specific image information to restrain copying etc. However, irrespective of whether the specific image information is discernible to the user or not, when for example, the user attempts to copy the document with the specific image information added thereto using a copying machine, etc., the machine performs an operation such as prohibiting the document from being copied or printed or outputting information of “copy prohibited”.
Various types of technologies are proposed as technologies for adding the specific image information into the document, for example, Japanese Laid-Open Patent Publication No. H07-038737 discloses a copying machine designed to easily prepare a document with a specific mark added thereto. In this patent application, the copying machine comprises a specific mark generating means for reproducing an image by adding a specific mark to a document image, a mark presence detecting means for detecting presence or absence of the specific mark in the read document image, and a controlling means for causing a copying operation to differ from the operation in an ordinary manner when the mark presence detecting means detects the specific mark from the document image.
Japanese Laid-Open Patent Publication No. 2000-307857 discloses printing of a candidate image of a specific pattern in a checkered manner on the back side of a document without impairing readability of the document. It also discloses judging of whether the detected candidate image of the specific pattern is the specific image information, by examining positional arrangement of the detected candidate image of the specific pattern and prohibiting an operation of copying, etc., in the case of the specific image information. Japanese Laid-Open Patent Publication No. H09-016043 discloses the technology of providing the specific image information divided into a plurality of colors and, at the time of detection, combining extracted results of the plurality of colors into one and accurately detecting the specific image information based on such results.
Copy prohibiting or detailed information on the read document is extracted by, generally, inclusive of the above prior techniques, adding (printing) a specific pattern image (specific image information) to image data of a document, reading the printed image data and detecting the specific image information from the read image data. In another instance, one document may include plural pieces of management information such as a serial number of a copying machine, a document manager, a copy available limit.
The specific image information embedded in image data has, however, limited color and size for the purpose of avoiding interference with the document image data, so that it is not easy to detect the specific image information added thereto with low detection accuracy, making it difficult to include a lot of management information.